testcode directives should support the same options as code-block

It's relatively easy to switch .. code-block:: directives out for .. testcode:: directives to make sure that documentation is up-to-date with respect to an implementation, which is great.

However, code blocks support several nice options that testcode blocks don't, making it impossible to substitute them 1-for-1 in all situations. For example, options like :caption: and :name: seem both very useful and like they should hopefully be easy to support, and the closer to parity the two directives are, the easier it will be to switch between them as desired.